1. Field of the Invention
The present invention relates to a sheet processor for processing a folded portion of a folded batch of sheets, such as a portion equivalent to a spine of a booklet, and to an image-forming apparatus provided with such a sheet processor.
2. Description of the Related Art
Conventionally, stitching/folding devices are used to form booklets by folding back a batch of about 20 or less stacked sheets. In such stitching/folding devices, a batch of sheets may simply be folded back, or may be saddle-stitched before being folded back, or may be bound together using an adhesive (i.e. perfect binding) instead of using thread or staples.
However, as shown in FIG. 15, the folded region of a batch S includes a folded portion Sb (i.e. the spine of a booklet) and its adjacent portions that form a curve and thus bulge. Since this bulging folded region of the batch S has resiliency, when multiple batches S of sheets are stacked one on top of the other, the stack of batches S becomes unstable. This led to difficulties in storing or transferring the stack of batches S.
To solve this problem, Japanese Patent Laid-Open No. 2001-260564 (GB2381237) (corresponding to U.S. Pat. No. 6,692,208), for example, discloses a sheet processor that can flatten the folded region of the batch S including the folded portion Sb so that the folded batch S can be laid flat, as shown in FIG. 16.
FIGS. 17A to 17C and 18 illustrate a conventional sheet processor 901. In the sheet processor 901, a half-folded saddle-stitched batch S of sheets forming a booklet is conveyed to a stopper plate 905 by a conveyor belt 906 in a manner such that the folded portion Sb (the spine) of the batch S is the leading end. Referring to FIG. 17A, the stopper plate 905 is movable in the vertical direction and temporarily receives and stops the batch S conveyed by the conveyor belt 906. Subsequently, referring to FIG. 17B, the batch S is nipped between a pair of gripping members 902 and 903, and the stopper plate 905 is then lifted upward. Here, the folded portion Sb protrudes from the pair of gripping members 902 and 903. The stopper plate 905 moves away from the folded portion Sb. The sheet processor 901 then presses a pressing roller 904 against the folded portion Sb and moves the pressing roller 904 along the folded portion Sb. Thus, the bulging folded portion Sb is pressed by the pressing roller 904 and is made flat, as shown in FIG. 18.
In the conventional sheet processor 901, the gripping members 902 and 903 nip the region of the batch S adjacent to the folded portion Sb entirely along the folded portion Sb before moving the pressing roller 904 to flatten the folded portion Sb. For this reason, the nipping force of the gripping members 902 and 903 lacks uniformity since the nipping force received by the folded portion Sb is different between the opposite longitudinal end portions of the folded portion Sb and the central portion of the folded portion Sb. Specifically, nipping entirely along the folded portion Sb in the longitudinal direction causes the gripping members 902 and 903 to bend, thus causing the opposite longitudinal end portions of the gripping members 902 and 903 to apply a stronger nipping force than the central portion. Consequently, when the folded portion Sb is being pressed and flattened by a folded-portion flattening unit of the sheet processor 901, a rip or a wrinkle can be formed quite easily at the central portion of the folded portion Sb where the nipping force received is relatively small.
Furthermore, because the gripping members 902 and 903 nip the region of the batch S adjacent to the folded portion Sb entirely along the folded portion Sb according to the sheet processor 901, a loose area in the folded portion Sb, which can be formed when the pressing roller 904 applies pressure to the folded portion Sb, cannot escape. Such a loose area forms a wrinkle in the folded portion Sb, thus leading to a bad appearance of the batch S.
U.S. Pat. Nos. 2,088,904 and 2,066,620 disclose devices for squaring the folded portion of a batch of sheets. Such devices are provided with a first roller for pressing against the folded portion of the batch of sheets, and a pair of second rollers for nipping a region of the batch adjacent to the folded portion. According to the concept of such a structure, the finished result of the processed folded portion of the batch is considered to be dependent on, for example, the positional relationship between the first roller and the second rollers, and the balance of the pressures applied to the batch of sheets from the first and second rollers. However, there are no disclosures in U.S. Pat. Nos. 2,088,904 and 2,066,620 regarding a structure for holding the first and second rollers.